This invention related to a method for preferentially hydrogenating the terminal methylene group attached to a tertiary carbon atom in an organic compound.
It is common knowledge in chemistry that unsaturated bonds present in organic compounds are converted to saturated bonds by various methods such as contact hydrogenation, treatment with composite metal hydrides and reduction with nascent hydrogen. Terminal methylene groups which are attached to tertiary carbon atoms are hydrogenated with difficulty.
.alpha.-Alkylvinyl groups, for example, are hydrogenated with great difficulty as compared with .alpha.-unsubstituted vinyl groups. Even when they are hydrogenated in the presence of powerful nickel type hydrogenation catalyst such as Raney nickel, the hydrogenation must be carried out under a high pressure on the order of some tens of atmospheres or at an elevated temperature of more than 100.degree. C. In the case of a compound which contains another unsaturated bond, however, when the hydrogenation is performed by such a method, the hydrogenation occurs additionally on the other unsaturated bond as well. By any of the methods mentioned above, therefore, the terminal methylene grop can neither be hydrogenated preferentially over the other unsaturated bonds nor be hydrogenated solely. An example of the sole hydrogenation of a terminal methylene group is found in a method for the manufacture of cumene by the hydrogenation of .alpha.-methylstyrene. By this method, the hydrogenation is carried out in the presence of palladium carried on activated carbon or rhodium carried on activated carbon as a catalyst under mild conditions of room temperature and normal pressure.
Since the catalyst used in the hydrogenation is very expensive, the method is not readily applicable to commercial production of cumene.
As described above, the only existing catalysts which permit the methylene group attached to a tertiary carbon atom in an organic compound to be preferentially hydrogenated over other unsaturated bonds or to be hydrogenated without affecting the other unsaturated bonds, without requiring the reaction system to be put under harsh conditions, are those based on expensive noble metals.
An object of this invention is to provide a method for hydrogenating the terminal methylene group attached to a tertiary carbon atom in an organic compound preferentially over the other unsaturated bond, under mild conditions.